Conventionally, when a wellbore is created, a number of casings are installed in the borehole to prevent collapse of the borehole wall and to prevent undesired outflow of drilling fluid into the formation or inflow of fluid from the formation into the borehole. The borehole is drilled in intervals whereby a casing, which is to be installed in a lower borehole interval, is lowered through a previously installed casing of an upper borehole interval. As a consequence of this procedure the casing of the lower interval is of smaller diameter than the casing of the upper interval. Thus, the casings are in a nested arrangement with casing diameters decreasing in downward direction. Cement annuli may be provided between the outer surfaces of the casings and the borehole wall to seal the casings from the borehole wall.
As a consequence of this nested arrangement a relatively large borehole diameter is required at the upper part of the wellbore. Such a large borehole diameter involves increased costs due to heavy casing handling equipment, large drill bits and increased volumes of drilling fluid and drill cuttings. Moreover, increased drilling rig time is involved due to required cement pumping, cement hardening, required equipment changes due to large variations in hole diameters drilled in the course of the well, and the large volume of cuttings drilled and removed.
At the surface end of the wellbore, a wellhead is formed that typically includes a surface casing, a number of production and/or drilling spools, valving, and a Christmas tree. Typically the wellhead further includes a concentric arrangement of casings including a production casing and one or more intermediate casings. The casings are typically supported using load-bearing slips positioned above the ground.
Conventionally, a wellbore casing cannot be formed during the drilling of the wellbore. Typically, the wellbore is drilled and then the wellbore casing is introduced in the newly drilled section of the wellbore. This delays the completion of a well. Moreover, the time it takes to retrieve the drill string and to subsequently introduce the casing may be longer than the time it takes for a part of the wellbore wall to collapse. Collapse of the walls of the wellbore is relatively expensive. As the time to retrieve the drill string increases with increasing depth of the wellbore, the risk of a collapsing wellbore wall also increases with increasing depth.
This risk is also significant during the creation of a sidetrack of an existing wellbore. Sidetracks may extend the life of a wellbore, by extending into the oil-bearing formation at an angle with respect to the original wellbore. In addition, when creating a sidetrack of an existing wellbore, the curvature of the sidetrack may prevent the introduction of a liner, thus limiting the depth and/or configuration of the sidetrack.
It has been proposed to overcome the problem of stepwise smaller inner diameters of wellbore casing by installing a tubular element in a wellbore and thereafter radially expanding the tubular element to a larger diameter. The tubular element may be expanded by means of an expander, which is for instance pulled, pushed or pumped through the tubular element.
WO-03/036025 provides a system for lining a section of a wellbore with an expandable tubular element. The system comprises an elongate drill string extending into the wellbore. The tubular element in unexpanded form encloses a lower portion of the string. The string is provided with an expander at the lower end of the tubular element. After drilling a new section of the wellbore, the expander is pulled upwards through the tubular element, thereby expanding the tubular element. An upper end of the tubular element extends into a lower end of the wellbore casing. Anchoring means including radial expansion means radially expand the upper end part of the tubular element against the casing.
The drill string passes on the relatively great forces involved in pulling the expander through the expandable tubular element. If the outside of the tubular element is provided with anchors for anchoring the tubular element to for instance the wellbore wall or a previous liner or casing section, the required (tractive) force is even greater at or near the location of the anchors, as the anchors must be introduced in the material surrounding the tubular element.